Torrefaction is a thermal pre-treatment technology used to upgrade lignocellulosic biomass to a higher quality and more attractive biofuel. In the torrefaction process, biomass is heated to 250-350°C in a low oxygen atmosphere, so that all moisture is removed and the biomass is partly devolatilises leading to a decrease in mass. Most of the initial energy content is preserved, so that the energy density of the torrefied biomass becomes in pellet or briquette form is higher than other solid biomasses in trade. Consequently, transportation and handling of torrefied pellets is much cheaper than wood pellets.

The typical mass and energy balance for woody biomass torrefaction is that 70% of the mass is retained as a solid product, containing more than 85% of the initial energy content. The other 30% of the mass is converted into torrefaction gas which contains up to 15% of the energy of the biomass. Ideally, the energy contained in these released volatiles equates to the heating requirements of the process. A thermal efficiency of around 95% can thus be achieved.

The properties of the final product highly depend on the process conditions and on the composition of the biomass feedstock. Depending on factors such as time, temperature and residence time, the biomass can be torrefied to different torrefaction degrees/temperatures. Directly connected to the degree of torrefaction is the net calorific value (NCV) of the resulting product. Theoretically, NCVs of 28 + MJ/kg could be reached, even though the overall process efficiency seems to be best at 20-22 MJ/kg NCV (depending on feedstock).

Most types of biomass contain hemicellulosic and cellulosic polymers. For this reason, torrefaction can be performed on virtually any lignocellulosic type of biomass, and it is possible in theory to design a torrefaction plant for a wider diversity of feedstock to produce a more homogeneous product. In this respect, torrefaction can also offer an opportunity for cheaper feedstock such as by-product streams, forestry or plantation residues or agricultural-material. However, the chemical composition of the biomass material is a factor to consider. Because of the relatively low temperature of the torrefaction process, most critical chemical fuel components (alkali metals, chloride, sulphur, nitrogen, heavy metals and ash) remain in the fuel after torrefaction. This makes clean biomass feedstocks the preferred option for the foreseeable future.

Besides the chemical composition, the physical characteristics of biomass play an important role when assessing the potential for torrefaction, biomass bulk density and content of hemicellulose, cellulose and lignin.

Torrefaction results in a high quality fuel with characteristics comparable to coal, as the table below illustrates. The increase in calorific value is caused by the removal of moisture and some organic compounds from the original biomass. A fundamental difference with charcoal is the difference in volatile matter; in torrefaction processes, the aim is to maintain volatile matter (and thereby energy) in the fuel as much as possible.

Why Torrefaction?

The torrefaction step represents an additional unit operation in the biomass utilisation chain. The consequential capital and operating costs, as well as conversion losses are, however, off-set by savings at the end use. Besides the economic potential, torrefaction brings many important advantages by overcoming traditional limitations of biomass while keeping the CO2 neutrality advantage:

Significant cost reductions in transport and handling resulting from a significant increase in energy density and water resistance

Broader feedstock basis – both geographically and in types of raw material

Almost no biodegradation of product when stored

Large variety of applications

Significantly increased grindability

Superior water resistance to wood pellets

Combusts cleanly, gasifies more easily

Can be produced to meet clients’ requirements

Reduced CO2 footprint along the supply chain

Helps develop the biomass market towards commoditization

Costs and the Technology Choice

There is an array of technologies to roast/dry/devolatilise biomass. In the same way as in other steps in biomass processing, a number of reactor designs are available for torrefaction. Screw conveyers, rotary drums (eventually combined with microwaves), moving beds, vibrating belts or torbed designs are the most common reactor types implemented. The choice of the reactor type and integrated technology is influenced by the feedstock and volumes to be processed, location of implementation and local electricity costs.

While an efficient and flexible torrefaction reactor is necessary, it is not sufficient for a torrefaction technology to become successful. Additional variables to consider are process integration, overall energy and mass efficiency, utilisation of all the energy from the feedstock and lastly mechanical compression efficiency and product quality.

In the framework of a cautious efficiency and sustainability setup, the energy balance of a torrefaction plant will only be marginally worse than that of a wood pellet plant. However, some extra heat losses and electricity consumption will appear. This is also reflected in slightly elevated processing costs in respect to wood pellet production.

The graph below illustrates the potential savings along the value chain compared to costs caused by wood pellets. It is based on identical feedstock to be processed.

While torrefaction represents an additional step in the processing of biomass with extra operating costs, there are savings elsewhere in the supply chain which offset these costs. The advantages of torrefaction are particularly salient in the logistics and storage areas. The additional investments related to torrefaction represent less than the associated logistic cost reduction. In fuel preparation at the power plant, the torrefied material behaves like coal. Milling or co-milling in existing coal mills is possible in many cases, as proven in test burns. Therefore, little new infrastructure is needed to co-fire torrefied pellets or briquettes with coal – a significant advantage especially at power plants which have not yet invested in wood pellet infrastructure.

Figure 3: cost torrefied vs white pellets. Source: Michael Wild

A typical setup of an integrated torrefaction plant consists of a feedstock pre-processing unit (shredding, chipping etc.), a drying unit, the torrefaction reactor, a product milling and a mechanical compression unit (pelleting, briquetting). In most cases, energy supply is integrated in the plant setup to also consume torrgases. Drying and torrefaction is in some cases carried out within one machine.

All this is valid for stand-alone torrefaction plants, of which the job it is to convert low-cost raw biomass into an internationally tradable product. Alternative setups may include torrefaction reactors integrated in the pre-processing line of a coal power plant, simply treating biomass prior to its milling in the coal mills or the torrefaction of already pelletized biomass. The latter benefits from the established pelleting technology or products available on the market, but will not improve transportation cost of wood pellets. If consumers are close to the torrefaction plant the delivery in form of powder might be a cost optimal solution.

A number of technology developers and suppliers have overcome the technical and integration difficulties and have excellent control of the entire process. Optimisation is done in the area of outdoor storage.

State of the Play

After several years of technology development, demonstration plant operation and test burns, a number of renowned companies have excellent control of the entire process are have started to roll out the technology as systems or component suppliers. First industrial-scale projects are implemented. Most of this initiative can be found in Europe ( Torrcoal, CEG, Arigna, Bioendev, Blackwood etc) or North America (RBE, TSI, AIREX etc), yet Asian companies have recently become very active in torrefaction (VEG..), creating also additional value to the large stream of agricultural by-products in the region.

Further optimisation of the process is ongoing, in cooperation with consuming partners. Further large scale testing in power plants and heat applications is required to adjust some of the parameters so that the final product can perfectly fit the consumers’ final requirements. The minimum requirements for torrefied or thermally treated biomass are now set as Technical Specifications.

Optimisation is taking place in the mechanical compression of pelleting or and improvement of water resistance (weatherability). However, there is not yet found an agreement by the market actors what water resistance does mean and how to measure it.

The availability of product to the market and the continuous production of torrefied biomass at several sites by a couple of producers created the basis for standardisation of the product. Within the ISO 17225 solid biofuels a Technical Specification for thermally treated biomass shall be published within weeks (ISO 17225-8).

Test and trial volumes of torrefied biomass are available from several producers, in several shapes and at different torrefaction levels.

Torrcoal plant in DIlsen Stokkem(B), AIREX plant in Becancour (QC)

Experience in Co-Firing Torrefied Biomass with Coal

Since 2012, test burns in European power plants have been undertaken, thereby co-combusting several thousands of tons from different producers. Although individual results are not to be published in detail, the outcomes from these test burns can be summarised as follows:

A number of successful test burns of torrefied biomass at European power plants

Co-firing from 5% to 25% and up to 85% (mass) and more

Superior characteristics of torrefied fuel were acknowledged

The performance of the material in the mill and in the burners was excellent, significantly better than that of white pellets

The emissions performance was satisfactory – no difference to wood pellets

Burner performance was excellent, better than any wood pellet fuel to date. No need for support fuels (needed with wood pellets)

Fuel preparation costs app. 0,48 €/GJ less than wood pellets

Improvements in performance of the torrefied material in the transfer system expected (dust formation, full water resistance)

Experience with Torrefied Biomass in non power applications

Although the power sector has long been the major focus for the consumption of torrefied biomass more and more heat applications open up for torrefied biomass as do small scale CHP option. In parallel to this rather small scale applications a number of industries is looking into torrefied biomass as source for their process energy, namely the cement, steel and non metallic minerals industries doe see a good potential to reduce their greenhouse gas profile by substituting todays conventional fuels by torrefied biomass.

The fact that torrefied biomass ships and stores more compact than any other solid biomass is providing significant advantages the longer the supply chain is. The almost smoke-free combustion is particularly of interest as is the preferential gasification properties of torrefied biomass. Ongoing and completed testings´ in furnaces and district heating boilers were as successful as were gasification trials in existing and dedicatedly build installations. Torrefied biomass here is not only traded in form of pellets but also briquettes of 50, 75 or 100mm diameter are brought to the market. Ireland currently leading in this application.

For this markets the publication of the Technical Specifications within ISO 17225-8 is even more important than for the power sector. Publication of the TS is expected by Q4 2016 or Q1 2017 and may boost developments in this market area.

Raw Material basis for Torrefaction

Each piece of solid biomass can be torrefied, not only wood. But can each biomass, once torrefied become an advantageous fuel. Mostly Yes. The torrefaction process is in most cases driving out some of the organically bound salts like Chlorine by this making especially Cl rich biomasses from agriculture a valuable resource basis. However, as the ahs characteristics of a raw material are not changed by torrefaction, not all agro biomasses will become an acceptable fuel.

Through torrefaction the path towards cheaper biomasses which are not in competition with food market or wood and timber markets is paved. Today unutilised by-product streams or processing of grasses will decrease costs and the impact of biomass consumption on sustainable development making it not only a carbon neutral fuel but also likely one of the most sustainable fuels available.

Trading Torrefied Biomasses

While the industry has become confident in the technologies and production capabilities, numerous activities concerning trading permission, logistics regulation as well as health and safety regulation were set.

The said TS under ISO 17225 has been initiated and received, a standard MSDS for torrefied biomass has been developed by the SECTOR project and IBTC jointly IMO testing on Flammablility and on Self-Heating Properties by VTT have proven that torrefied biomass is not to be classified as flammable solid material or as a self-heating substance according IMO rules

Technical/safety characteristics of the torrefied dust do not differ significantly from those of normal biomass dust, but are clearly more reactive than coal dust. The brittleness of torrefied biomass does, however, result in good chances to produce more dust in handling and, therefore, extra care shall be taken in not allowing dust to accumulate.

Regarding trade in Europe, a consortium of five companies has been established with the assistance of IBTC, in order to clarify the exemption possibility or registration necessities under REACH regulation with ECHA, the European Chemicals Agency.

IBTC: A Cooperation Platform to Accelerate Progress

The International Biomass Torrefaction Council (IBTC) was founded in 2012 as a platform for companies, organisations and individuals dedicated to the promotion of torrefied biomass for energy.

The most important torrefied biomass producers are today Full Members of the group which includes 22 members.

The platform allows the discussion of common interests not under competition and facilitates companies´ synergy to overcome the barriers that hinder the market development.

The main objective of IBTC is to promote the use of torrefied biomass as energy for the power and the heat market, undertake studies and projects to increase the depth of knowledge, gain permissions eventually needed for the trade of torrefied products and to spread the concerns of the industry to the outside world. Michael Wild is the President of the group which is coordinated by Cristina Calderón of AEBIOM.